Allow for specifying I/O variables using Vensim names in spec file

[chrispcampbell]

Currently SDE expects the inputVars and outputVars arrays in the JSON spec file to be provided as C names with numeric indices, e.g. _var_name[0][1]. This requires advance knowledge of subscript->index mapping and generally makes usage more difficult.

We should allow for providing these variables using the original Vensim names instead, and convert to C names as needed inside of SDE. We can call the arrays inputVarNames and outputVarNames, and these will take precedence over inputVars and outputVars.